Micro-electro mechanical systems (MEMS) continue to improve in functionality. New uses and applications for such devices continue to be discovered. MEMS can contain both mechanical structures and integrated circuit (IC) type electrical components. For example, very small mechanical switches, mirrors, sensors, can be manufactured using silicon manufacturing technology. Generally, devices of such MEMS can be manufactured in large quantities at low cost, making them cost-effective for many uses.
There is a group of MEMS based devices built with a stationary frame that have at least one beam (possibly a cantilever beam) that connects a “movable” member to the stationary frame. As stated above the moveable portion or member can be a switch contact, a mirror, an accelerometer, an optical switch utilizing movable mirrors, a probe storage, or MEMS memory device, and a cell sorting device to name a few. In some MEMS, it is necessary to integrate at least one electrical component on the movable member. In these systems, one must provide electrical connections to these components. In one type of system, probe storage devices will have a movable member or movable structure that can be utilized to provide motion of a ceramic or semiconductor tip such as an atomic force microscope (AFM) tip, working as a read-write head, with respect to a memory/data storage material.
Mechanical properties of suspension structures and moveable members and their dependence on time, temperature and mechanical load are very important design constraints. The mechanical configuration and materials utilized in each design can significantly change the functionality and performance of a MEMS device. Designers often try to provide suspension arrangements with very stable mechanical properties with large lifetimes, wide temperature ranges and high reliability under a wide range of mechanical loads. Time dependence of mechanical properties of suspension arrangements, as for example, time dependence of suspension spring constant can cause short-term and long-term instability of parameters of MEMS devices. Temperature dependence of suspension parameters can cause significant temperature dependence of parameters of MEMS. Significant dependence of suspension properties on mechanical load can cause non-linear effects in MEMS. All the above types of instability are highly undesirable in most applications.
It can be appreciated that an electrical connection between the stationary frame and the movable portion or movable structure is often required and can be established by different means. For example, an electrical connection can be provided through the body of the substrate either using conductivity of bulk material or with help of highly conductive layers formed within the body of the wafer. In such cases, the body of the suspension beam can be utilized as a conductor or a low-resistive layer can be created within the suspension beams using, for example, ion implant or silicidation. An electrical connection can also be formed between the frame and the movable structure by depositing conductive layers on top of the suspension. Different metals, poly-silicon, and other conductive materials, as for example, titanium nitride can be used for this purpose. Suspension beams also can be made out of such conductive material.
The above described approaches are less than perfect and have many drawbacks and deficiencies. For example if the electrical connection to the movable structure is established through the bulk of the wafer, typically very few electrical connections can be made, as for example, a power connection and a ground connection. Providing more than one connection can require electrical isolation between different conductors. Although this task can be solved, for example, by using SOI material, forming suspension beams in the device layer and isolating the conductors from each other by trenches etched through the device layer, these approaches are costly and require either special material or complex manufacturing processes. Therefore, designs utilizing electrical connections through the bulk of the substrate have limited application.
Placing a metal conductor on top of the suspension beams as well as using conductive layers within body of the wafer can create a “bi-metallic” configuration, where different layers have different coefficients of thermal expansion and temperature changes can make the beams bend either up or down depending on whether the temperature rises and falls. Such temperature dependent bending can cause significant temperature dependence of parameters of MEMS devices, which is highly undesirable in most cases. Besides that, deposited layers of metal can create built-in stress in the beams from this deposition of conductive material. This stress can change with time and also after exposure to temperature cycles. For example phenomenon known as stress relaxation can cause undesirable changes in the suspension parameters. This change in parameters of MEMS device can cause thermal hysteresis, fracture and fatigue among other things. Therefore, this “sandwich” approach is also less than perfect.
Using suspension structures that are manufactured out of conducting material that is different from material of the wafer, is also less than perfect because of the stress gradients, which are typically present in the deposited metal layer(s). The stress gradient can be result from metal deposition processes such as sputtering, evaporation, plating, etc. The stress gradient can cause bending and other deformation of suspension beams. Beams with built-in stress gradient also can undergo other phenomena such as stress relaxation making suspension properties unstable in time and dependent on temperature.
As stated above, one technical challenge is that often a movable structure can require numerous conductors and thus, numerous conductors need to be provided from the frame to the movable structure. In some cases-conductors should be capable of carrying significant current. Since the suspension beams are relatively narrow in many cases, typically only a limited number of conductors such as one or two conductors can be placed on top of a suspension beam. Increasing the number of suspension beams is undesirable in many cases as this may cause significant changes in mechanical properties of the suspension. In particular, increasing the spring constant of the suspension beyond an acceptable limit can adversely affect the entire design. Therefore, providing the required number of electrical connections to the movable part of MEMS devices can be a challenging task.